Field of the Invention
Embodiments of the invention relate to a flow meter or a water cut meter (hereinafter collectively referred to as a “flow meter”) for measuring water cut and salinity levels of a multiphase mixture, and more particularly to a flow meter including a coaxial probe array which determines in real-time the presence of multiphase flow in a mixture and water cut and salinity levels of a multiphase mixture having a high salinity concentration.
Description of the Related Art
Real-time water cut and salinity measurements of multiphase mixtures are highly demanded by daily operations for both reservoir management and production allocation in the oil and gas industry. Conventional flow meters are limited by water cut measurement ranges and the level of salinity found in multiphase mixtures. For example, reservoir and completion brines are commonly observed to have salinity levels greater than the salinity of sea water, which typically has a salinity level of about 35 parts per thousand (ppt). In some cases, the salinity level of reservoir and completion brines range from about 100 ppt to about 200 ppt, and has been observed at a saturation concentration of 360 ppt. Connate water has also been commonly observed to have salinity levels greater than the salinity of sea water. In some cases, the salinity level of connate water has been observed at around 200 ppt.
The measurement of permittivity (i.e., relative dielectric constant) is a means for estimating the aqueous phase(s) of a multiphase mixture. In particular, permittivity measurement using capacitance or microwave sensors is a common process for estimating water cut and salinity of the multiphase mixture. Capacitance is observed to increase between a pair of electrodes as the amount of water between the electrodes increases. The effect of increasing the salinity of the water in a multiphase mixture is to introduce a parallel conduction path between the electrodes which inhibits a measurement of capacitance at low frequencies where the imaginary component of permittivity exceeds a value of 300. Thus, capacitance-based water cut measurements are observed to fail at high levels of salinity. As a result, salinity variation causes the inaccuracy of water cut measurements, preventing well production optimization within the capacity constraints of production facilities.
Electromagnetics sensors have been designed to more accurately measure the water cut or water-in-liquid ratio (WLR) of multiphase mixtures (e.g., oil-water or oil-water-gas multiphase fluids, as non-limiting examples). In some industrial and scientific applications, electromagnetic coaxial probes have been used to measure water cut in both single phase and multiphase mixtures encountered in oilfields, based on the principle that the measured complex reflection coefficient (i.e., ratio of reflected signal to the incident signal) is dependent on the aperture impedance (i.e., complex permittivity) of a sample material terminating the probe.
One such sensor includes a single microwave, open-ended coaxial reflection probe which measures the permittivity and conductivity of a multiphase mixture and combines the measured mixture permittivity and conductivity with a known or derived relationship between brine water conductivity and brine water permittivity to estimate the on-line brine water conductivity of the mixture. The probe is mounted at an opening of a production pipe wall, and thus only measures a small subset of the overall cross-section of the production pipe (i.e., a small measurement area of the permittivity and conductivity of the multiphase mixture).
Another such sensor includes one or more open-coaxial probes, operating at about 1 GHz, arranged along the diameter of, or affixed to a column in, a production pipe. Each probe detects an amplitude and a phase of a reflected signal for identifying single phase and multiphase mixtures based on the differences between the real part of each mixture's complex permittivity. Each probe requires a set of electronics to measure the complex permittivity of the respective mixture.
In order to effectively optimize well production within the constraints of production facilities, flow meters providing more accurate water cut and salinity measurements for multiphase mixtures having high salinity concentrations will need to be developed for the oil and gas industry.